Can chess, not a physically demanding sport, really be so difficult for its aging champions?
Absolutely! The rating lists provide conclusive evidence that the ability to play chess at a very high level declines sharply after age 50. Even those rare grandmasters who remained successful in their mid-50s, such as Viktor Korchnoi and Sammy Reshevsky, were markedly weaker than in their prime years. For most grandmasters, the drop is between 100 and 200 rating points. Others, though, retire rather than let their rating plummet.
From Some grandmasters are older and wiser, but weaker.